Organic solvent- and/or water-based liquid flexo or gravure inks (“solvent-based printing inks”) have been widely used in various types of printing, such as packaging, using inkjet printer. The printed images should have good solvent and abrasion resistance. At the same time, it is necessary for solvent-based printing inks to have good re-solubility to avoid ink drying and clogging on the plate, anilox and gravure cylinders of the printers. Typically, low molecular weight (MW) resins and plasticizers offer good re-solubility, yet, in order to improve resistance properties of the printed images, higher MW resins are required. This represents a major contradiction and problem for the formulators of the solvent-based packaging liquid inks.
To solve these problems, various types of photocurable printing inks having specific combinations of photocurable resins and photoinitiators, have been developed (see, for example, U.S. Pat. Nos. 4,066,582; 4,221,686; 4,303,924; 5,057,398; and WO 01/57145 A1). However, incomplete polymerization of these inks often results in diffusion of uncured resins and causes smears or abrasion of the printed images.
As pointed out in U.S. Pat. No. 6,528,127, the printing of thermoplastic packaging films has remained a bit of a black art despite advances in general printing techniques. Packagers have recently been requiring film manufacturers to provide packaging films bearing photograph quality printed images, which is difficult in and of itself, sometimes for uses which add to the difficulty. Heat shrinkable films and thermoforming films are particularly challenging because of the need for the printing ink(s) to resist cracking or flaking off once the film has undergone heat shrinking, a process during which the film is subjected to heat, friction, and/or film-to-metal contact. Films intended for cook-in applications can undergo all of these strenuous conditions and provide film manufacturers and converters with some of their greatest printing challenges.
To prevent cracking and/or flaking of printed images, film manufacturers tried several strategies. Most often, these involve the use of new ink formulations to avoid using standard inks containing pigments carried in a resin which is soluble in a solvent such as an alcohol which must be evaporated, leaving behind the resin-pigment combination. The newer formulations have involved two-part polyurethane resin systems as well as solvent-free systems in which the resin(s) can be cured by means of ultraviolet (UV) light, which also have drawbacks, including concerns regarding operator exposure to components and the need to assure sufficient cross-linking to comply with applicable governmental food safety regulations.
To solve these problems, U.S. Pat. No. 6,528,127 teaches a manufacturing process for food packaging materials comprising printing the plastic film with solvent-based inks which are dried without electron beam or ultraviolet radiation, followed by applying a radiation-curable, pigment-free protective coating and radiation curing the coating. In this process that represents an advance, it has been found that an excessive amount of residual solvent is trapped in the ink under the protective coating and causes excessive odor from the packaging material. In addition, since drying rate of solvent varies significantly over multiple heating units as well as the number and amount of ink traps, the cure and friction characteristics of the finished packaging material can be non-uniform throughout the printed images. This, in turn, can cause problems with processing the packaging material in the filling lines, particularly in relation to coefficient of friction (COF, which represents frictional resistance between two surfaces and can be expressed as static COF, where the surfaces are static, and kinetic COF, where the surfaces are in motion) and/or slipping issues. For example, a job printed at a customer with three solvent based inks on the polyethylene (PE) film and over-printed with electron-beam (EB) curable coating, has the following coating-crosslinking cure patterns: (i) coating over PE film—30 MEK (methyl ethyl ketone) rubs (see the definition below); (ii) coating over white ink—8-10 MEK rubs; (iii) coating over yellow and white inks—3-4 MEK rubs; and (iv) coating over black, yellow and white inks—only 1 MEK rub. Thus, the EB coating cures differently over different color inks primarily because the solvent-based inks release their solvent differently. The thicker the underlying ink layer, the slower the cure of the overlaying energy-curable coating. Variations in degree of cure result in variations in COF value. Thus, there is a need for a printing method for, in particular, food packaging, that addresses these problems while providing an imaged film with the desired characteristics.